For example, a capacitance-changing type humidity sensor is described in U.S. Pat. No. 6,742,387 (corresponding to JP-A-2003-156464). This humidity sensor is used for detecting humidity within a room to perform humidity control of an air conditioner, or is used for detecting humidity of the outdoors to make a weather observation.
When it is simply called humidity, there is generally relative humidity and absolute humidity. In the relative humidity, the ratio of an amount of water vapor actually included within the atmosphere, to a maximum amount of water vapor (saturated water vapor pressure) able to be included in the atmosphere at its temperature, is shown by percentage. In the absolute humidity, the amount of water vapor included within the air of one cubic meter at one atmospheric pressure is shown by a gram number. The humidity sensor described here is normally used as a sensor for measuring the above-described relative humidity.
In this humidity sensor, for example, a silicon oxide film as an insulating film is formed on a silicon substrate. A pair of electrodes each of which has a comb teeth shape is formed on the silicon oxide film so as to be separated and opposed to each other on the same surface. A silicon nitride film as a protecting film is formed on this pair of electrodes. A humidity sensing film having a capacitance value changed in accordance with humidity is formed on the silicon nitride film so as to cover both the electrodes.
Here, an area for forming the humidity sensing film is constructed as a humidity sensing portion for sensing humidity. When moisture enters the interior of the humidity sensing film, the dielectric constant of the humidity sensing film is greatly changed in accordance with the entering moisture amount because the dielectric constant of a water molecule is large. As a result, the capacitance value between both the electrodes is also changed. In the humidity sensing portion, because the capacitance value between the pair of electrodes is changed in accordance with the humidity change around the sensor, humidity can be detected on the basis of this capacitance value change.
As mentioned above, in the humidity sensor for detecting the indoor humidity as a sensor for the humidity control of the air conditioner, or used for detecting the outdoor humidity for the weather observation, etc., the humidity can be appropriately detected on the basis of the dielectric constant change of the humidity sensing film, i.e., the capacitance value change between the pair of electrodes.
On the other hand, in recent years, it is desired to more accurately measure an intake air amount supplied to an engine so as to optimize an air-fuel ratio in electronic control fuel injection of the engine. As a method for measuring such an intake air amount, there are a mass flow system (L-EFI) for directly measuring the intake air amount by using e.g., an air flow meter, and a speed density system (D-EFI) for combining a pressure sensor and a temperature sensor and presuming the air amount supplied to the engine per one cycle by an engine rotation number and an intake pipe pressure. However, in each of these systems, because no water vapor included within the air can be separated from the air, its unseparated amount becomes an error. Accordingly, a method for detecting the humidity together with the intake air amount and more accurately measuring the air amount by this humidity detection is considered to aim at the engine control of higher precision in the future.
However, when the humidity sensor is arranged in an intake gas system of the engine, e.g., within an intake manifold, etc. so as to accurately measure the intake air amount in this way, the following disadvantages newly become matters unable to be neglected. That is, the interior of such an intake manifold lies under a deteriorated environment in which various polluting substances such as engine oil, carbon, fuel, etc. exist. Accordingly, when the humidity sensor is arranged under such an environment, there is a fear that such polluting substances are attached to the humidity sensing portion of the humidity sensor. If such polluting substances are attached to the humidity sensing portion, the flow of the moisture through the humidity sensing film is obstructed. Therefore, responsibility of the humidity sensor is extremely reduced. Further, because no dielectric constant of the polluting substance can be distinguished from that of the humidity sensing film, a shift is caused with respect to a measuring value of the humidity sensor.
Such a problem is also caused in a composite sensor considered to be particularly useful in the speed density system (D-EFI) and having a humidity detecting function provided by combining e.g., a pressure sensor, a temperature sensor, etc. with the humidity sensor.